Physiological functions, such as the electrical activity of the heart or instantaneous blood pressure measurements, are generally expressed as analog waveforms signals on strip chart recorders and CRT displays. For signal processing reasons, these signals are generally digitized prior to display. Unfortunately, digitization usually occurs at a rate which is greater than the display apparatus can accommodate such that the sample-rate of the wave-data must be adjusted to facilitate displaying the waveform on a discrete imaging medium, such as a bit-mapped CRT or LCD screen, or on a dot-matrix printer.
A typical an analog-to-digital converter may output digital data samples corresponding to some type of electrical activity at the rate of 3000 samples per second. If the display device can only drive 100 pixels (samples) per second, a thirty-to-one digitized data sample to representative display sample compression ratio is required. One way of providing the required compression of the data in this situation would be to average each successive group of thirty data samples. Unfortunately, this reduces the amplitude of narrow pulses to the point where they may become indistinguishable from noise and results in the loss of signal features.
U.S. Pat. No. 4,499,548 discloses a "peak-picking" technique for compressing data in which the absolute difference between the value of each digitized data ample in a group of data samples and the average value of the previous group of digitized data samples is determined. The data sample which has the largest absolute difference is selected as the representative display sample. In another embodiment, the data sample which differs the most from the data sample that was selected as the display sample in the previous group is selected as the representative display sample. The principle limitation of this technique is the loss of feature-amplitude-fidelity at high compression-factors. With high compression-factors, peak-picking attenuates or even obliterates parts of poly-phasic features of waveforms. For example, a bi-phasic pace-pulse may be represented as a mono-phasic pulse in a peak-picked waveform.
Cascading is a compression technique in which a data stream is compressed and the resulting data is compressed again. Peak-picking is unsuitable for cascading as severe distortion may arise upon averaging a number of consecutive samples. After the first application of the method, the sample-stream contains only peaks, and averaging them no longer provides a valid criterion for sample selection.
"Pair-picking" is a known technique which maintains this feature-amplitude-fidelity for most compression-factors by plotting a vertical line segment between the maximum and minimum data sample in each group of data samples. The ordered-sample-pair output format employed by this technique is convenient for drawing waveforms, but inconvenient for other applications of the data. Integer compression factors can be applied to generate one sample-pair to be mapped directly onto each column of pixels in a bitmap or to define the limits of each scan of a digital-recorder print-head to produce a waveform image.